There is disclosed in DE-A-31 13 110, a rod lens which is of a smaller diameter centrally than at its ends. Rod lenses so shaped are supported in the internal tube merely at their end regions, so that bending of the tube, which is unavoidable when it is in use, does not cause the rod lenses to be stressed to an extent to result in their breakage. Nevertheless, the cemented joints between the rods and the correction lenses are subjected to shear stress sufficient for risk of their fracture to arise. According to DE-A-34 31 631, cylindrically shaped rod lenses, are supported and centered by spacer rings arranged in the internal tube or on the rod lenses themselves. The rod lenses are supported, however, in the regions where the correction lenses are cemented thereto, so that, in this case also, the cemented joints are subjected to shear when the internal tube is bent.